In recent years, hybrid vehicles and electric vehicles have been rapidly developed. In line with this, various suggestions have been made to change the braking system of vehicles from the conventional mechanical hydraulic control to electrohydraulic control.
A battery is generally used as a power supply for controlling vehicles electrohydraulically. However, if the power supply from the battery is cut off for some reason, it may be impossible to hydraulically control and hence to brake the vehicles.
To prepare for such an emergency, various vehicle power supply devices have been proposed which include high-capacity capacitors or the like as an auxiliary power supply besides a battery.
Vehicle power supply devices, which involve the vehicle braking in an emergency, are essential to supplying electric power surely in an emergency. Therefore, it is required to ensure the accurate determination of degradation of capacitors, which are key devices for the vehicle power supply devices.
A conventional vehicle power supply device determines degradation of the power supply as follows. First, the vehicle power supply device determines the internal resistance value and the capacitance value of the capacitor unit including a plurality of capacitors. The device then makes a temperature sensor provided near the capacitor unit detect the temperature in the vicinity of the capacitor unit. The device then corrects the internal resistance value and the capacitance value based on the detected temperature. The device compares these corrected values with the degradation standard value data corresponding to the detected temperature.
When sufficient electricity is supplied to the load, the internal resistance value of the capacitor unit is correlated with the inverse of the capacitance value. However, the correlation changes when the capacitor unit is degraded.
The characteristic that the correlation changes in this manner is taken advantage to determine the degradation of the vehicle power supply device. More specifically, the relation between the internal resistance value and the capacitance value, after the capacitor unit is degraded, is previously stored with respect to each temperature in a ROM (Read only memory) connected to a controller (microcomputer). The vehicle power supply device is determined to be degraded when the internal resistance value corresponding to the capacitance value, which is corrected based on the temperature at a certain point, reaches the stored degradation standard value of the internal resistance value.
An example of the degradation standard value data of the internal resistance value corresponding to the capacitance value is shown in FIG. 10. In FIG. 10, the horizontal axis represents the capacitance value, and the vertical axis represents the internal resistance value. The correlation between the capacitance value and the internal resistance value differs depending on the temperature. The correlation is shown at temperatures at 15° C. intervals between −30° C. and 30° C.
When a capacitor unit has a temperature of 0° C., a capacitance value of 10 F, and an internal resistance value of 130 mΩ, the degradation standard value (limit value) at 0° C. can be obtained in FIG. 10. In short, as shown by the circle plots (0° C.) of FIG. 10, degradation standard value 101 is an internal resistance value of 230 mΩ. Accordingly, the capacitor unit has not reached standard value 101, so that it is determined not to be degraded yet.
When a capacitor unit has a temperature of 15° C., a capacitance value of 11 F, and an internal resistance value of 115 mΩ, the degradation standard value at 15° C. can be also obtained in FIG. 10. In short, as shown by the square plots (15° C.) of FIG. 10, degradation standard value 102 is an internal resistance value of 180 mΩ. Accordingly, the capacitor unit has not reached standard value 102, so that it is also determined not to be degraded yet.
On the other hand, when a capacitor unit has a temperature of 30° C., a capacitance value of 11 F, and an internal resistance value of 110 mΩ, the degradation standard value at 30° C. can be also obtained in FIG. 10. In short, as shown by the “X” plots (30° C.) of FIG. 10, degradation standard value 103 is an internal resistance value of 80 mΩ. Accordingly, the capacitor unit has exceeded standard value 103, so that it is determined to be degraded.
One such conventional vehicle power supply device is disclosed, for example, in Japanese Patent Unexamined Publication No. 2005-28908.